1. Field of the Invention
The present invention generally relates to a method for fabricating planar light waveguide circuits and, more particularly, to a method for fabricating planar light waveguide circuits with a vertical taper structure.
2. Description of the Related Art
In general, a planar light waveguide circuit (PLC) is an essential component of the integrated optical circuit used in a high-speed optical network. Its application extends to an optical distributor, coupler, modulator, interferometry switch, semiconductor laser, and circuit for use in the dense-wavelength-division-multiplexing (DWDD) communication.
FIG. 1 is a perspective view of a known planar light waveguide circuit. As shown in the drawing, the planar light waveguide circuit includes a slab 111, an arrayed waveguide 112, and a branch section for making a junction with the arrayed waveguide 112. However, during operation, the mode mismatch of the branch section between the slab 111 and the arrayed waveguide 112 tends to cause optical signal loss. That is, many optical signals are lost primarily because the pluralities of the arrayed waveguides 112 and the slab 111 have different-sized branches. For example, the coupling of the slab 111 of arrayed waveguide gratings and arrayed waveguide 112, or the mode mismatch of the Y-Branch of the optical-power splitter and arrayed waveguide 112 cause cross-talk with other optical signals of different channels, thus losing optical signals in the course.
FIG. 2a through FIG. 2c are perspective views representing each step of the fabrication process of the conventional planar light waveguide circuit with a vertical taper structure. Referring to FIG. 2a through FIG. 2c, a planar light waveguide circuit 220 includes an arrayed waveguide 222, a slab 221, and a branch for connecting the slab 221 and the arrayed waveguide 222. The branch has a vertical taper structure 223 that is formed using a shadow mask 250. That is, the branch connecting the slab 221 and the arrayed waveguide 222 in the planar light waveguide circuit 220 includes a vertical taper structure 223 with a decreasing tilt in a fan-shape along the longitudinal direction of the arrayed waveguide 222.
As FIGS. 2a through 2c manifest, the method for fabricating a planar light waveguide circuit 220 with the vertical taper structure 223 using a shadow mask 250 involves layering a hard layer 210, forming a mask pattern 211, 212 on the hard layer 210, installing a shadow mask 250, and dry-etching the core layer 220.
Referring to FIG. 2a, the mask pattern is formed by layering the hard layer 210 on the core layer 220, and then forming a mask pattern to be shaped on the hard layer 210. Usually, a metallic or inorganic thin film is used for the hard layer 211. Underneath the core layer 220 is a clad layer 230.
Referring to FIGS. 2b and 2d, the shadow mask is installed by first placing a shadow mask 250 with a designated height 252 at the branch between the slab 221 and the arrayed waveguide 222 of the planer light waveguide circuit, and then installing the shadow mask 250 on the upper portion of the dummy mask 251. This procedure determines the tilt (or slope) of the vertical taper structure 223. That is, the tilt, length, or shape of the vertical taper is adjusted by adjusting the height 252 of the dummy mask 251 and the size 253 of the shadow mask 250.
Nevertheless, problems remain in the vertical taper structure using the shadow mask in that it is not that easy to adjust the gap between the shadow mask and the planar light waveguide and to array the dummy mask precisely on the branch. As a result, reproducibility of the products is degraded, and, naturally, they are not appropriate for mass production. In addition, as shown in FIG. 2c, in the step of dry-etching the vertical taper structure 223 is adjusted by modulating the height 252 and the size 253 of the shadow mask 250, and its etching rate. A gas (mainly, ion plasma) is used for the dry-etching step to process fine patterns. The preferred gas for the dry-etching includes SF6, CF4, BCl3, Cl2, SiCl2 and so forth. As such, it is not easy to adjust shape (e.g., tilt and length) of the vertical taper structure.